This invention relates to optical fiber signal transmission and, in particular, to the generation of polarization-mode dispersion (PMD) to emulate the natural occurrence of PMD in optical fiber; and to use PMD emulation to compensate for PMD generated by optical fiber.
Polarization-Mode Dispersion (PMD) is a fiber-optic telecommunication system impairment which can prevent the transmission of high data rates, such as 10 Gb/s and 40 Gb/s. The effect of PMD originates with the inherent, built-in residual birefringence present in all single-mode optical fiber. Over the course of pulse transmission, PMD interacts with a transmitted optical pulse in such a way as to distort the shape of the pulse. The consequences vary with the degree of pulse distortion, from small penalties in transmission fidelity, to complete system outage. Accordingly, in order to transmit optical pulses at rates above 2.5 Gb/s, either the quality of the optical fiber must be sufficiently high so as to not introduce significant PMD or a PMD-compensating apparatus must be inserted in the transmission system, for example, between the end of the fiber-optic transmission line and the input to the optical receiver.
In order to transmit at a rate of 10 Gb/s over legacy fiber (that is, currently installed fiber), a PMD compensator (PMDC) is frequently necessary to recover acceptable system performance. It is generally believed that in order to transmit at a rate of 40 Gb/s and above over the most recently available fiber, a PMDC at the receiver may be essential. Accordingly, a PMDC is a desirable apparatus. A means for laboratory and factory testing of a PMDC is, consequently, a desirable apparatus. Such an apparatus is herein referred to as a PMD Emulator (PMDE).
Indeed a PMDC and PMDE are quite similar because both apparatuses must generate PMD; the former apparatus must generate PMD in order to cancel the accrued fiber PMD, the latter apparatus must generate PMD in order to test a PMDC. However, the generation of PMD for a PMDC and PMDE does have practical differences. A PMDC further requires the generation of a control signal which is used to monitor the PMD cancellation, and further requires a feedback system and control algorithm which automatically corrects for changing PMD. A PMDE further requires the precise and repeatable generation of PMD, and may not require the speed of change which may be necessary for a PMDC. A PMDE further requires the synthesis of the PMD effect so as to approximate the PMD of a real fiber as closely as possible. A PMDE further requires a performance which is both known and repeatable so as to test and verify the performance of a PMDC.
The term xe2x80x9cPMDCxe2x80x9d will refer herein to an apparatus which consists of: 1) a PMD generating mechanism; 2) a control-signal generating mechanism; and 3) a feedback control mechanism and algorithm which changes the PMD generating mechanism so as to cancel the PMD of the fiber-optical link. The term xe2x80x9cPMDExe2x80x9d will refer solely to a PMD generating mechanism, which includes a means to change the state of PMD. It is recognized that a PMDE can be transformed into a PMDC through the addition of a control-signal generating mechanism and a feedback control mechanism and algorithm.
Polarization-mode dispersion is the composite phenomenon of two interleaved effects. One effect is the projection of an input state-of-polarization (SOP) onto a birefringent dielectric system. The other effect is an accrued differential temporal delay between two orthogonal polarization states. FIG. 1a illustrates an input optical pulse 100 with an arbitrary input SOP 120. The pulse 100 is incident upon an optical birefringent medium 110 with orthogonal birefringent axes fast 121 and slow 122. The terms xe2x80x9cfastxe2x80x9d and xe2x80x9cslowxe2x80x9d refer to the speed of the optical pulse as projected on either axis: the pulse on one axis propagates faster than the pulse of the other axis due to the difference in refractive index, the latter which is due to the inherent birefringence of the fiber. The projection of the input SOP 120 onto the birefringent interface 110 results in the formation of two orthogonally polarized pulses 101 and 102. The balance of energy on the two orthogonal polarization axes is dictated by the relative orientation of the input SOP 120 and the birefringent axes 121, 122 at the interface 110. FIG. 1a illustrates the phenomena of polarization projection at a birefringent interface.
FIG. 1b illustrates an example of xe2x80x9csimplexe2x80x9d PMD. A short section of optical fiber 130 and the effect of PMD on an optical pulse 100 is herein illustrated. The optical pulse 100 has its SOP 120 projected onto the birefringent axes of the fiber 110, resulting in pulse 101 on fast axis 121 and pulse 102 on slow axis 122. The birefringence of fiber 130 causes a relative temporal delay between the two pulses 101 and 102. This temporal delay is referred to as differential-group delay (DGD). At the end of optical fiber 130 pulses 101 and 102 exhibit a DGD of magnitude xcex94xcfx84 140. The magnitude of DGD 140 depends on the magnitude of the birefringence and the length of fiber 130 over which the birefringence does not significantly change. The present instance of a single polarization projection followed by a single differential-group delay stage is denoted as simple, one one-stage, PMD.
FIG. 2 illustrates the concatenation of several simple PMD stages to form a more complex PMD response. FIG. 2a illustrates substantially the same PMD as FIG. 1b but the orthogonal polarization states are not explicitly indicated. FIG. 2a illustrates a pulse 200 input to birefringent fiber segment 130. The PMD of this fiber segment 130 generates DGD 240 between two output pulses 200 and 201. FIG. 2b illustrates the concatenation effect of two birefringent fiber segments 130 and 131 possessing dissimilar lengths and birefringent orientation. Fiber segment 130 produces two pulses 200 and 201 with DGD 240. Fiber segment 131 produces two pulses for each pulse input, resulting in four pulses 200, 201, 202, 203. The time delay between pulse images 200 and 202, and 201 and 203, is the DGD 241 of fiber segment 131. FIG. 2c adds a third fiber segment 132 with dissimilar length and birefringent axis orientation. Again, each input pulse 200, 201, 202, 203 to fiber segment 132 is copied and each pair 210, 211 is delayed by DGD 242, forming four pulse pairs 210, 211, 212, 213. Note that at each interface between fiber segments, the polarization projection alters the balance of energy between that on the incident SOP and that on the projected coordinates; thus, the variation in pulse amplitudes.
The fiber within a typical fiber-optical link is composed of tens or hundreds of fiber segments joined in series much as those in FIG. 2c. The time-domain representation becomes difficult to extend to such a fiber because of the geometric increase in the number of pulses that is output from a long fiber link. The appropriate alternative representation is in the frequency domain. FIGS. 3a and 3b illustrate the customary technical representation of the PMD effect in the frequency domain. The production of multiple pulses with various relative temporal delays is Fourier transformed into the spectrum of DGD, FIG. 3a. The magnitude of DGD 301 is plotted as a function of frequency 300. The relative energies of the output pulses and their composite state-of-polarization is represented by the Poincare-sphere representation of Principal States of Polarization (PSP). The PSP is used to represent the overall birefringent axes of a whole fiber link at each frequency. If an input sinusoidal optical wave has an SOP which aligns to the PSP of the fiber which corresponds to the frequency of the optical wave, then the energy of the input optical wave is completely transferred to only one PSP axis. Any other input SOP will cause a splitting of the input pulse energy onto the two orthogonal PSPs of the fiber. FIG. 3b illustrates the Poincare sphere 310, which is a suitable representation of states-of-polarization, and PSP 1 vector of one frequency, 320, and PSP 2 vector of another frequency, 321. The direction of the vector is the Principal State of Polarization at one frequency. The length of the vector is the DGD 301 at that frequency 300. The PSP of the fiber changes for each frequency, mapping a contour of PSPs 330.
FIG. 4a illustrates DGD spectrum 301 on frequency axis 300. The optical signal pulse spectrum 400 is indicated in relation to the DGD spectrum. Four frequencies are considered 401, 402, 403, 404. FIG. 4b illustrates the temporal delay between optical signals at each particular frequency 401-404. Note that this is illustrative, because an optical signal at one particular frequency is a sinusoidal wave and not a pulse; a pulse is used here figuratively. Consider frequency 401 and the result of DGD 301 on a pulse in time. On time axis 410 pulses 420 and 421 experience relative time delay 422 in accordance with the value of DGD at frequency 401 On time axis 410 pulses 423 and 424 experience a relative time delay 425 in accordance with the value of DGD at frequency 402. On time axis 410 pulses 426 and 427 experience a relative time delay 428 in accordance with the value of DGD at frequency 403. On time axis 410 pulses 429 and 430 experience a relative time delay 431 in accordance with the value of DGD at frequency 404. Each impact of distinct relative temporal delays 422, 425, 428, 431, for each frequency component of optical signal pulse 400 can cause significant pulse distortion. Note that in FIG. 4b all pulse heights are all equal. This is for illustrative purposes and does not show the complete effect.
FIG. 5a illustrates the PSP xe2x80x9cspectrumxe2x80x9d 330 on the Poincare sphere 310 as it may vary from PSP 1, 320, to PSP 2, 321, as a function of frequency. Four frequencies on the PSP spectrum 330 are indicated, 401-404. Each frequency is coincident with the frequency illustrated in FIG. 4a. The pulse input SOP vector is indicated 500. At each frequency 401-404 the input pulse SOP is projected onto the PSP vector. The projection results in a power rebalancing between two orthogonal PSPs. FIG. 5b illustrates the combined effect of DGD and SOP-to-PSP projection. Input SOP 500 is projected at frequency 401 in such a manner as to rebalance the pulse energies as indicated by pulses 520, 521. Pulses 520, 521 experience DGD 422. Input SOP 500 is projected at frequency 402 in such a manner as to rebalance the pulse energies as indicated by pulses 522, 523. Pulses 522, 523 experience DGD 425. Input SOP 500 is projected at frequency 403 in such a manner as to rebalance the pulse energies as indicated by pulses 524, 525. Pulses 524, 525 experience DGD 428. Input SOP 500 is projected at frequency 404 in such a manner as to rebalance the pulse energies as indicated by pulses 526, 527. Pulses 526, 527 experience DGD 431. The impact of distinct relative temporal delays at each frequency component of optical signal pulse 400, coupled with the energy rebalancing due to the SOP-to-PSP projection, can cause significant pulse distortion. FIG. 5b illustrates more fully the impact of PMD to an optical pulse.
Prior Art exists for a PMD emulator apparatus. FIG. 6a illustrates a method which concatenates several segments of highly birefringent fiber 601, 602, 603. In order to alter the state of PMD at the output, mechanisms 610, 611, 612, are attached along the fiber length which physically rotate the fiber about its longitudinal axis. Two or more fiber segments are used for this apparatus. The relative rotation of the fiber segments 601, 602, 603 changes the projection of preceding-segment output pulse SOP and following-segment input fiber birefringent axes. The DGD of each fiber segment 601, 602, 603 is fixed. The limitations of this embodiment include: the birefringence of the fiber segments 601-603 is not well controlled during construction and over temperature and aging; the rotation of one fiber segment, e.g. 602, relative to an adjacent fiber segment, e.g. 603, is limited to the torsional breaking point of the fiber, and hence rotation is not endless. In sum, the state of PMD is not easily determined in real-time and not easily repeatable.
FIG. 6b illustrates another apparatus for PMD emulation. Lithium-Niobate (LiNbO) waveguiding polarization controllers 620 transform the SOP from input 630 to output 631. Highly birefringent fiber segments 604, 605, impart DGD. The LiNbO polarization controllers utilize the electro-optic effect of the LiNbO crystal to alter the SOP of the incoming light 630. Electrodes 622 are driven by differential voltage 623 to impart an SOP rotation on waveguide 621. Multiple electrode stages 622 are employed to impart multiple polarization transformations. The limitations of this embodiment include: the actual degree and direction of polarization rotation is not easily monitored and may vary from device to device, the same will change with temperature and aging; the birefringence of the fiber is not well controlled during construction and may change due to environmental effects. In sum, the state of PMD is not easily determined in real-time and not easily repeatable.
FIG. 6c illustrates another apparatus for PMD emulation. A single LiNbO waveguiding polarization controller 620 is employed. Along the length of waveguide 621, between electrodes 622, there is a small degree of DGD which is inherently generated. The design of a device with sufficient number of stages provides for multiple SOP transformation stages and interleaved DGD stages. The limitations of this embodiment include: the SOP transformation from stage to stage is not easily monitored; the actual DGD generation may vary from crystal to crystal; the DGD sections are not easily distinguished from the SOP transformation sections 622. In sum, the state of PMD is not easily determined in real-time and not easily repeatable.
Prior-Art approaches to PMD generation suffer from one or more drawbacks, each of which are sufficient to limit their utility.
Therefore, it is the object of the present invention to provide a means and apparatus to generate the effect of PMD in a manner which is known, predictable, repeatable, and sufficient to approximate the behavior of optical fiber.
A polarization-mode-dispersion emulation (PMDE) apparatus in accordance with the principles of the present invention includes, briefly and generally, a plurality of PMD-generating stages all positioned to provide a clear light-path through each stage, wherein each PMD-generating stage further includes a first waveplate element, a first birefringent element, and a second waveplate element, all positioned to provide a clear light-path through each element in succession in the order herein listed. To alter the state of PMD which is generated by the apparatus, the waveplate elements are mounted on motorized rotation stages which are operated by a controller that coordinates the waveplate rotation about the axis perpendicular to the birefringent plane. Through rotation of the waveplate elements, the magnitude and modulation of the differential-group delay (DGD) spectrum of the generated PMD can be controlled.
The concatenation of PMD-generating stages provides for the interleaving of two complimentary optical effects which generate PMD: the projection of states-of-polarization onto orthogonal birefringent axes and a differential group delay subsequently generated. In accordance with the present invention, the state of PMD which is generated by the apparatus is controlled by the accurate rotation of waveplates, one or more which precedes each birefringent crystal, and by the accurate construction and selection of birefringent crystals. In one preferred embodiment of the present invention, the differential group delay generated by each birefringent crystals located within the apparatus is substantially of the same magnitude.
According to another aspect of the present invention, the insertion loss through the PMD-generating apparatus does not substantially vary with the rotation of the waveplates. Typically, the attachment of the waveplates to rotation stages does not provide for zero wobble of the waveplate over 360 degrees of rotation. Any residual wobble imparts displacement on the transiting optical beam which in turn generates rotation-dependent loss. As disclosed in the present invention, use of true zero-order single-plate waveplates minimizes rotation-dependent loss while still providing for the rotation of the state-of-polarization from PMD-generating stage to PMD-generating stage.
According to another aspect of the present invention, the accuracy, predictability, programmability, and repeatability of the generated PMD is a distinguishing feature of the disclosed PMD-generating apparatus. The utilization of birefringent crystals and waveplates, whose optical and mechanical properties are well known and stable, and the utilization of high-accuracy rotation stages provided with the measurement and recording of the rotation orientation, together facilitate the predictability of the generated PMD. Moreover, the replacement of the single birefringent crystals with composite crystals, wherein each composite crystal is designed to substantively eliminate temperature variation of the imparted differential-group delay, further enhances the repeatability of the inventive apparatus.
According to another aspect of the present invention, the PMD-generating apparatus of the present invention can be employed to test a PMD compensator (PMDC) apparatus. Through the change of PMD generated by a PMDE, the performance of a PMDC may be determined.
According to another aspect of the present invention, the PMD-generating apparatus of the present invention can be employed as one part of a PMD compensator apparatus, wherein the PMD-generating apparatus receives optical signals impaired by the PMD effect, a detector apparatus receives the optical signals distorted by both the original PMD effect and subsequently the PMD-generating apparatus effect, a detector monitor measures the degree of total PMD present on the optical signal, and a controller rotates at least one waveplate located within the PMD-generating apparatus so as to minimize the degree of total PMD present on the optical signal.
Additional objects, advantages, and features of the various aspects of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings.